The objective of the present invention is to use attenuation and velocity dispersion data to determine parameters that describe subsurface fluid distribution. These parameters may include spatial distribution fluids, heterogeneity length scale, and average saturation. Specifically, the invention employs a poroelastic rock physics model (and its frequency behavior or asymptotes) to provide a physics-based interpretation of geophysical parameters. Published work that might be relevant to the invention falls into two classifications. They are either theoretical studies or patents which seek to invert geophysical data into fluid distribution parameters, mainly average saturation. See U.S. Pat. No. 7,136,757 to Goloshubin and Korneev; U.S. Patent Application Publication No. 2008/0175099 by Hawthorne et al.; and Tserkovanyak and Johnson, 2004). There are many theoretical modeling studies that demonstrate aspects of the physical mechanism employed by the invention (see, for example, Johnson 2001; Pride at al. 2004; Toms et al. 2008), but none of these studies present any strategy to interpret the geophysical information for parameters describing the fluid distribution. The present inventors are not aware of any published work aimed at using attenuation to define all three significant aspects of subsurface fluid distribution (spatial distribution, length scale and average saturation).
In U.S. Pat. No. 7,136,757, Goloshubin and Korneev (2006) utilize the reflection characteristics of seismic waves from a target layer and reference layer of known average fluid saturation to determine the average fluid saturation within the target layer. The reference layer is normally decided from the borehole or log data; however other reference layers may be utilized. They perform a low pass filtering of the windowed reflections from the target layer leaving frequencies below the lowest recorded frequency spectra. They define a complex ratio R(x,ω)=W(x,ω)/W(x0,ω) of the low frequency amplitude spectra of the target W(x,ω) and reference layer W(x0,ω) data. They determine the average relative reflected amplitude A(x) by summing the ratios R(x,ω) of the first N reliable low frequency ω. They also analyze the derivative of the average amplitude DA(x) with respect to frequency and the derivative of the average time delay DP(x) with respect to frequency. They map the average fluid saturations by utilizing the calibration conditions: A(x0)=1, DA(x0)=0, and DP(x0)=0 at the location x=x0. The invention patented by Goloshubin and Korneev estimates only average fluid saturation and fluid type based on attribute (seismic spectral) variation from the calibration conditions. While attenuation and phase velocity dispersion may be implicit in the physics of this invention, in that the reflected amplitude from the target and reference layers will be affected similarly by velocity dispersion, this additional physical knowledge is not exploited to determine the full gamut of subsurface fluid distribution parameters.
In U.S. Patent Application Publication US 2008/0175099, Hawthorne et al. determine fluid type within subsurface formations surrounding a borehole. The invention utilizes sonic logging data, in addition to open-hole log measurements of porosity, lithology etc. They propose three different ways of characterizing fluid type. The first is to determine formation mobility, which is the ratio of rock permeability to fluid viscosity. They propose using the formation mobility to indicate fluid viscosity which differs for heavy oil, oil, gas and water. The second approach considers pore fluid bulk modulus, as the compressibility of heavy oil, water, gas etc is very different. The third approach considers attenuation. The invention utilizes compressional, shear and Stoneley waveforms, where slowness (the inverse of velocity) and attenuation of each waveform may be utilized to invert for pore fluid bulk modulus, borehole mud slowness or formation mobility.
Tserkovank and Johnson (2001) seek to invert geophysical data for heterogeneity length scale information. They utilize the laboratory experimental data acquired by Cadoret et al. (1998). They assumed a respective water saturation, and for each saturation used the corresponding P-wave attenuation and velocity measurements to determine a length scale for the heterogeneity. Their method employs a rock physics model that assumes a fixed fluid distribution pattern described by periodically distributed spherical saturation heterogeneities. The approach described in the Tserkovanayak and Johnson paper employs a straight inversion of attenuation and velocity information to determine length scale, utilizing a rock physics model knowing all other rock and fluid properties, including the average fluid saturation.